submodule--helper: move config-sanitizing to submodule.c

submodule--helper: move config-sanitizing to submodule.c

These functions should be used by any code which spawns a
submodule process, which may happen in submodule.c (e.g.,
for spawning fetch). Let's move them there and make them
public so that submodule--helper can continue to use them.

Since they're now public, let's also provide a basic overview
of their intended use.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>